The invention concerns an infrared (IR) microscope for a Fourier transform (FT)-IR spectrometer having a Cassegrain mirror-lens, with which an incident infrared luminous beam can be focused by means of a convex mirror configured rotationally symmetric with respect to the optical axis of the Cassegrain mirror-lens and by means of a concave mirror which is likewise rotationally symmetric to the optical axis of the lens under an angle of incidence .beta.&lt;60.degree. with respect to the optical axis of the lens after travelling along an optical path x onto a first point-shaped region on the surface of a sample, the region being simultaneously the hypothetical point of intersection of the optical axis of the lens through the sample surface.
An IR microscope of this type is, for example, known in the art from publication DE-OS 33 03 140.
The known IR microscope is utilized in an FTIR spectrometer for the analyses of gaseous, liquid and, preferentially, solid samples of macroscopic size. Thereby it is also possible to measure microscopic regions of the sample surface in a point-like fashion, whereby the measurements are carried out in reflection.
A disadvantage of the known IR microscope is that the IR luminous beam which is focused onto the sample surface is incident upon this surface at an incidence angle .beta.&lt;60.degree. with respect to the optical axis of the lens. In special applications, in particular when the interaction of the incident IR radiation with the sample surface must be intensified, a substantially larger angle of incidence .beta.' can be desirable in order to increase the usable measuring signals which, in an extreme case, would be in the vicinity of 90.degree. so that the incident IR luminous beam nearly grazes the sample surface.
Although there are special-grazing-angle microscopes such as for example, that known from the arrangement of publication DE-OS 37 04 239, these known apparatuses utilize lenses which are highly specialized and expensive and are adapted solely for the case of grazing incidence of the luminous beam onto the sample surface.
It is therefore the purpose of the present invention to present an IR microscope, which in large part corresponds to that described above, and distinguishes itself therefrom solely through a small number of simple and inexpensive optical components with which it can be reconfigured from the original steeper angle of incidence to a nearly grazing incidence of the IR luminous beam onto the sample surface, whereby the imaging properties should be maintained, the optical path lengths which the radiation travels in the IR microscope up to the sample should remain constant and whereby, subsequent to the reconfiguration, the infrared luminous beam incident on the sample surface should still remain focused.